On 9/28/2011 11:22 AM, Koji Ishii wrote:
> If you look at font tables[1], they match to their input systems; MS Gothic, Meiryo, and Kozuka Gothic on Windows has vert feature for U+2015 but not for U+2014, while Hiragino on Mac has vert feature for U+2014 but not for U+2015.
My take is that the 'vert' feature is ill-defined, but that we need to
implement a decent layout on top of it, the way it is today. (there are
other similar cases, e.g. U+00B0 ° DEGREE SIGN, which do not have the
added complication of the confusion between U+2014 and U+2015).
My proposal is essentially to restrict the application of 'vert' to
those cases where it is absolutely necessary (e.g. square katakana,
ideographic stop, etc), and in particular not use it for the dashes
where it's not that useful. Hopefully, all the common fonts have done
(in the same way) the cases where it's absolutely necessary to go
through a feature, and differ only on those cases where we don't really
need to apply 'vert'.
In more concrete terms, the UTR 50 draft now has three orientations: U,
S, and T (for Transformed). T is for square katakana symbols, the weavy
dash and a handful of others. The T are defined purely in terms of
Unicode. Think in terms of Unicode code charts: What the charts show
today is a representative glyph for horizontal lines. Let's say we want
to produce the code charts for vertical lines: most cases will the same
glyph either Upright (=as is) or Sideways (= rotate 90 degree
clockwise), but a few are Transformed.
Not surprisingly, the 'vert' feature does the same thing on Ts in all
fonts (well, all those that support vertical Japanese text). So
restricting 'vert' to the Ts gives a reliable system.
This will also give us decent layout. We can get great layout by adding
a new feature, which will be restricted to change the shape of glyphs
but not incorporate a rotation aspect. Because of that restriction, this
new feature can be applied liberally and give a lot of freedom to the
font designer, without putting the layout engine in a loose-loose situation.
Note that 'vert' does not have this restriction of "free to change the
shape, but do not rotate". This is precisely what makes it so hard to
use when it's not done the same across fonts. Since we cannot come back
on that, we have to either use 'vert' only in the most restricted
circumstances, or to react to whether it actually changed the glyph
(which I find way too problematic).
Eric.